Non-destructive methods for characterizing damage caused by hidden corrosion and fatigue in layered structures such as aircraft lapjoints are a high priority for commercial airlines and the military. Eddy current instruments have long been used for non-destructive testing. Eddy currents penetrate into subsurface layers whether or not the layers are mechanically bonded, which is an advantage over ultrasonic techniques that require a mechanical bond between layers for the ultrasonic energy to penetrate to deeper layers.
Eddy current testing instruments include a probe that is positioned on the surface of a structure to be inspected. A coil in the probe is electrically excited, either by a sinusoidal or a pulsed signal, and a response is measured. Pulsed excitation causes the propagation of a highly attenuated traveling wave, which is governed by the diffusion equation, which states that the diffusive propagation of the eddy current pulse results in spatial broadening and a delay, or travel time, proportional to the square of the distance traveled.
Pulsed eddy current (PEC) methods have important advantages over sinusoidal methods. The pulsed eddy current method excites the probe with a step voltage function. The step function contains a continuum of frequencies. As a result, the response to several different frequencies can be measured with a single step. Because depth of penetration is dependent on the frequency of the excitation, information from a range of depths can be obtained with the one step function.
One PEC inspection instrument that has been developed employs pulsed eddy currents for characterizing corrosion-induced loss of metal in aircraft structures. See Moulder et al., "Pulsed eddy-current measurements of corrosion-induced metal loss:theory and experiment," Review of Progress in Quantitative NDE, Vol. 14, pp. 2065-2072 (Plenum Press, NY, 1995, Thompson et al. eds.). This instrument calculates and displays to the user a difference signal which is the difference between a time-domain PEC pulse response acquired by the PEC probe and a time-domain reference PEC pulse response. The user is able to see from the magnitude of the displayed difference signal whether there is metal loss associated with corrosion in the inspected sample structure. This instrument detects metal loss in all layers of a multi-layer structure.
Another PEC instrument is one that has been developed by the UK Defense Research Agency. See Harrison, "The detection of corrosion in layered structures using transient eddy currents," Nondestructive Testing of Materials, pp. 115-24 (IOS Press, 1995, R. Collins et al., eds.). This PEC instrument scans a probe across a layered structure and obtains time-domain magnetic-field transient responses. To obtain what is referred to as a "balanced transient," each of the responses is subtracted from a transient response obtained from an initial position. For each balanced transient, several amplitude time slices, ten in particular, are measured. The article cited above discloses that different timeslices may be displayed which gives information about different depths of the inspected structure. An amplitude window can also be set to obtain a color-coded display of only time-slice amplitudes within the set amplitude window.